What is the embryology and anatomy of the ear and where does otitis externa (OE) occur?

Updated: Mar 09, 2020
  • Author: Ariel A Waitzman, MD, FRCSC; Chief Editor: Ravindhra G Elluru, MD, PhD  more...
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Answer

The external ear (see the image below) consists of the auricle and the EAC.

Acute otitis externa. Ear canal is red and edemato Acute otitis externa. Ear canal is red and edematous, and discharge is present.

The auricle is composed of elastic cartilage with the overlying skin attached directly to the perichondrium. It begins to form during week 6 of gestation through consolidation of portions of the mesoderm of the first and second branchial arches, giving rise to the His hillocks. The first three hillocks are derived from the first arch, the second three from the second arch. The auricle reaches adult shape by the week 20 of gestation, but the adult size is not reached until the age of 9 years.

The EAC begins to form during week 8 of gestation, when the surface ectoderm of the first pharyngeal groove thickens and grows toward the middle ear. This core of tissue begins to resorb by week 21 of gestation to form a channel that is complete by week 28. The canal reaches adult size by the age of 9 years and ossifies completely by the age of 3 years. The EAC is related to the mandibular fossa anteriorly, the mastoid air cells posteriorly, the middle cranial fossa superiorly, and the parotid gland inferiorly.

The EAC is lined with squamous epithelium and is approximately 2.5 cm long in adults. Its function is to transmit sound to the middle ear while protecting more proximal structures from foreign bodies and any changes in environmental conditions. The outer one third of the canal is primarily cartilaginous and is oriented superiorly and posteriorly; the inner two thirds of the canal is osseous, is covered with thinner skin that adheres tightly, and is oriented inferiorly and anteriorly; this portion of the canal is devoid of any apocrine glands or hair follicles.

The thicker skin over the outer (cartilaginous) portion of the EAC contains apopilosebaceous units comprising apocrine and eccrine glands that secrete their products around the base of a hair follicle. These secretions combine with sloughed squamous epithelium (cerumen) to coat the EAC and maintain an acidic pH (4-5). This cerumen coat migrates from the isthmus of the EAC to the lateral part, and its waxy nature protects the underlying epithelium from maceration or skin breakdown. The quantity of cerumen produced varies widely among individuals.

The acidity of the cerumen inhibits bacterial or fungal growth. Whereas a paucity of cerumen allows bacterial growth, an excess can create an environment ideal for bacterial invasion by allowing retention of water and debris (as when the EAC is regularly exposed to water). Localized trauma from foreign objects placed in the ear can also lead to direct bacterial invasion in the ear canal. Once an infection becomes established, localized maceration and inflammation occur, which lead to symptoms.


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